Numerous papers devoted to problems of reducing fire hazard of polymeric materials have so far been published. For that purpose flame retardants of various compositions, such as inorganic, halogen- and phosphorus-containing compounds, are used.
Researchers are paying a great deal of attention to the problems of ecological safety of fire-protection materials. A number of firms carry out research work aimed at developing flame retardants and fire-protection technology for polymeric materials that are safe for the environment; in particular, halogen free flame retardant systems are being developed. Toxicological aspect of the problem has been studied extensively, and new FR substances and compositions for polymeric materials have been developed that reduce flammability of the latter and at the same time demonstrate low toxicity and smoke emission.
However, despite the extensive research work performed, the task of reducing flammability of polymers together with minimising smoke emission and combustion product toxicity during pyrolysis of polymers has not yet been solved completely.
Initially, halogenated organic compounds were used for reducing flammability of polymers, mainly aromatic brominated flame retardants, due to their high thermal stability and lower smoke emission compared to aliphatic halogenated compounds. Effectiveness of FR effect of the said flame retardants increases when they are applied together with metal oxides, predominantly with antimony trioxide. In order to reduce smoke emission special additives are used together with the above mentioned systems, most active of which are oxides of aluminium, zinc, and tin [cf. Cusack P. A., 1991, v. 32, #2, pp. 177–190].
However, though smoke emission in the presence of halogen-containing flame retardants may be reduced, the problems of corrosiveness, toxicity, and low resistance to UV-radiation of the compositions obtained are very acute.
The use of halogen containing flame retardants in polymeric materials is restricted due to ecological reasons. Decomposition of polybrominated phenyl ethers gives off dioxines and furanes, which have adverse effect on the ozone layer in the atmosphere thus limiting their application as flame retardants.
Phosphorus containing flame retardants are, to a greater extent, free from the aforementioned disadvantages; in addition, FR effect of phosphorus is three to four times that of bromine (at equal concentrations) (cf. Aseyeva R. A. Combustion of polymeric materials, Moscow, Nauka PH, 1981, p. 249 (in Russian).
To reduce flammability of polymers using phosphorus-containing compounds three principal techniques are applied:
adding flame retardants to a polymer melt;
surface treatment of fabrics and fibres;
chemical modification of polymers.
Adding flame retardants during polymer processing is the most widespread and efficient method of FR protection of polymeric materials, since this method does not require new equipment and is economically effecient. However, application of this method is limited by requirements to a FR additive, i.e. it should be thermally up to 300° C., should dispense easily, have a suitable melting point and high degree of dispersion.
Red phosphorus is a fairly efficient FR for polycaproamide and is used in combinations with metals (cf. Povstugar V. I. Structure and properties of the surface of polymeric materials, Moscow, Khimia PH, 1988, 192 pages (in Russian).
Glass-filled polyamides 6 and 66 modified with a phosphorus-vanadium-containing FR system (with 3–4% phosphorus content in the composition) had the FR class of V—O with the limiting oxygen index of 28–30%. However, high toxicity and fire hazard of red phosphorus, as well as complexity of the technological process, restricted practical application of the said method.
A number of studies (cf. Levchik G. F. Thermochim. Acta, 1995, v. 257, pp. 117–121) is devoted to reducing flammability of polycaproamide using ammonium polyphosphate (APP) in combination with inorganic additives, in particular, talc, CaCO3, ZnCO3, MnO2.
A disadvantage of this method is in high levels of added to polycaproamide APP—at least 30%, which affects adversely physical and mechanical characteristics of the polymer.
Much attention is given to reducing flammability of polycaproamide by using melamine and its derivatives—cyanuric and isocyanuric acids in combination with metals or phosphorus containing flame retardants (cf. Levchik G. F., Polym. Degrad. Stab., 1996, v. 54, pp. 217–222).
Adding 30% of melamine-isocyanurate to the polymer melt gives a polycaproamide composition with LOI of 27%.
However, as it has been pointed out in a number of papers, adding melamine derivatives to a polycaproamide melt results in higher brittleness of the resultant compositions.
In industry the improved FR performance of polyester (PE) is achieved by adding an oligomer derivative of phenylphosphonic acid (Bisphenol-S, available from Toyobo, Japan, and Eni-Chem, Italy) to the PE melt during molding (cf. Horrocks A. R. Polym. Degrad. Stab., 1966, v. 54, pp. 143–154).
Of great interest for reducing flammability of polyethyleneterephthalate (PETP) is a cyclic phosphonate available from Albright and Wilson under the trade-name of Amgard 1045 (cf. Horrocks A. R. Polym. Degrad. Stab., 1996, v. 54, pp. 143–154). This flame retardant may be used both for finishing textile material and as an additive during processing. Modified fabrics and fibres have LOI of 26–27%.
Most widespread and readily available phosphorus-containing FRs for polyolefins are APP and combinations of APP with polyatomic alcohols and/or melamine (cf. Gnedin Ye. V. High-molecular compounds. Series A, 1991, v. 33, #3, pp. 621–626 (in Russian). These flame retardants are foaming ones, that is, forming highly porous carbonised chars with low heat transfer properties. The use of foaming flame retardant systems increases FR characteristics of polyethylene (PE) and polypropylene (PP). However, for preparing compositions with FR class V—O, FR loading levels should be at least 30%. In addition, in the course of high-temperature processing of compositions containing foaming systems, their components start reacting thus giving off gases that complicate the processing and affect physical and chemical characteristics of the resultant materials.
Micro encapsulated in a polyurethane sheathing APP-FR CROS-484 EC—is available from Bolid GmbH (Germany) (cf. Catalogue of flame retardants. Bolid GmbH, Frankfurt a.M., 1996, 21 pages) is recommended for FR protection of various polymers, including polyolefins.
In the paper by Zubkova N. S. (Plastics, 1996, #5, pp. 35–36) it was proposed to use a micro encapsulated complex of a nitrogen derivative of methylphosphonic acid and ammonium chloride as a flame retardant for polyethylene and polypropylene. However, adding this FR system to polymer melts is difficult because of destruction of microcapsules and evolution of gases, which affect stability of the polymer processing.
A method of surface treatment of polymers which is a further one for reducing their flammability consists in fixing a dissolved, emulsified, or suspended flame retardant additive on a fiber or fabric. The method is comparatively simple and allows of using flame retardants that make part of the composition of finishing agents.
Said composition for fire-protecting finishing may comprise, apart from flame retardants, also promoters, dispersers, dyes, latexes, and so on. To fix flame retardants on a fabric the latter is treated in the presence of methylol compounds or melamine-formaldehyde resins by drying an impregnated fabric at 60–100° C. or heating at 160–170° C. for 2–3 min.
A highly extended class of additives is applicable for imparting fire protecting properties to polymers using said surface-treatment method, comprising phosphorus- and phosphorus-nitrogen-containing compounds, polyphosphates, and some other organic compounds (cf. Sharma V. N., Colourage, 1979, v. 26, #7, pp. 27–33).
Phosphorus-containing flame retardants most frequently used for surface treatment of polycaproamide materials are orthophosphoric acid and esters thereof in combination with epoxy compounds or melamine-formaldehyde resins (cf. FRG Application #3,622,840, IPC C 08 K 5/52).
Most widely used for surface treatment of polycaproamide materials is tetra-(hydroxymethyl)-phosphonium-chloride (THPC) (cf. U.S. Pat. No. 4,750,911). Treatment is carried out jointly with trimethylamine and carbamide under conditions involving heat treatment at 130–140° C., with the resultant undissolvable cross-linked product formed on the fabric surface. A fire-protecting effect is attainable with at least 25% fabric weight increment. However, the burning process of a fabric treated with THPC is accompanied with evolution of toxic phosphine.
Ciba-Geigy AG (Switzerland) produces a composition Pyrovatex-CP recommendable for finishing polyester fabrics and those from a mixture of cellulose and polyester fibres (cf. Shama V. N. Colourage, 1979, v. 26, #7, pp. 27–33). Said composition comprises N-methylol-(O,O-dimethylphosphon-hydroxypropionamide). However, efficiency of fire-protecting effect produced by said composition for fabrics containing above 15% PETP is rather low, because said flame retardant is liable to decompose at a lower temperature compared with that of starting thermooxidative destruction of the original polymer.
The authors of the paper by Camino G. Polym. Degrad. Stab., 1988, v. 20, #3–4, pp. 271–294) propose treating a fabric from PETF and a mixture thereof with cellulose fibres with a composition “Proban” comprising tetra(hydroxymethyl)-phosphonium chloride and polyfunctional nitrogen-containing compounds. However, said composition, similarly to the preceding one, possesses but a low fire-protecting efficiency for fabrics comprising more than 15% of the PETF fibres. To produce fabrics from PETF fibres having reduced flammability, it is recommended that such fabrics be treated with the “Proban” composition at least twice, accompanied additionally with partial phosphorus oxidation into a pentavalent form by treating the dried fabric with an aqueous hydrogen peroxide.
U.S. Pat. No. 4,732,789 discloses a two-stage method of treating PETF fabrics, comprising impregnating the fabric first with the “Proban” composition, then with hexabromocyclododecane or a cyclic phosphonate. Next the thus-treated fabric is subjected to thermofixation; when use is made of hexabromocyclododecane, the fabric is to be heated above 182° C. for the flame retardant to melt down. However, the presence of two stages in the proposed process and a necessity to effect thermofixation of the fabric at high temperatures impede much practical application of the proposed method.
Liquid phosphorus-bromine-containing compositions known under trade name of Antiblaze 315, 345 (cf. Brossas J. Polym. Degrad. Stab., 1989, v. 23, #4, pp. 313–326) are proposed for treating PETF fabrics and finishing textile materials from man-made fibres.
However, the aforementioned compositions may be used for modifying decorative and drapery materials alone, while hardly flammable fabrics may be obtained with a fabric weight increment of at least 30–40% which affects adversely the feel of the fabric and deteriorates physical and chemical characteristics of the materials.
Chemical modification of polymers is performed both at the stage of synthesis thereof and at the stage of finished material and enables flammability of materials to be reduced by changing the structure and properties of macromolecules. Such a modification is most frequently used in the course of polymer synthesis. Said method is in widespread use for treating fireproof PETF fibres (Shovka N. Text Research J., 1993, v. 63, pp. 575–579). There are available from the firm Hoechst in the Federal Republic of Germany fireproof PETF fibres and filaments marketed under various trade marks and produced by copolymerization with phosphorus-containing monomers.
Most widespread are Trevira fibres (FR and CS) used for manufacturing fireproof children's sleeping clothes, upholstery, drapery, and industrial fabrics, as well as curtains an carpets. The fibres are readily dyeable in bright rich hues and are resistant to the effect of direct sunrays.
However, fireproofing characteristics of said fibres are but inadequately high so that with the phosphorus content within 08 and 1.0% the oxygen index equals 26–27%.
With a view to obtaining PETF fibres having reducing flammability, the authors of the paper by Ma Z., J. Appl. Polym. Sci., 1997, v. 63, pp. 1511–1515) propose synthesizing a phosphorus-containing copolymer capable of forming, under the effect of heat flows, a volumetric carbonised layer, i.e., foamed char possessing good heat-insulating properties. Used as a phosphorus-containing copolymer was a phosphorus-containing pentaerythrite derivative. Adding 10% of a phosphorus-containing flame retardant to the PETP polymer chain increases the oxygen index of the composition up to 28%.
A finished polymer can also be modified chemically by treating said polymer with various chemical agents. To attain high-degree fireproofing, it is necessary to conduct chemical conversions featuring high-degree substitutions, which has an adverse effect on the properties of the resultant materials.
Surface modification of polymeric materials is more economical than the volumetric one, thus presenting a promising way for reducing flammability of many kinds of materials, such as fibres, fabrics, and films.
French Patent #93 08466 discloses the use of a salt of alkylaminomethylenephosphonic acid as a flame retardant for impregnating cellulose fibres and fibres from a mixture of cellulose fibres and polyester fibres; this salt has the following formula: 
where: R is CH3 or C2H5;                H+ is hydrogen cation;        A+ are the cations of nitrogen-containing compounds, such as dicyandiamide, guanidine, carbamide and its derivatives;        B+ is ammonium cation;        x is 0 to 6, y is 0 to n, Z is 0 to n;        x+y+Z=2n.        
The aforementioned flame retardants were prepared by mixing a respective acid with one of the abovelisted nitrogen-containing compounds in an aqueous medium at a temperature of from 20° C. to the temperature of the salt precipitation. The salts of the abovementioned formula are produced in an aqueous solution with a concentration of from 30 to 60%. For treating cellulose-base fabrics and fabrics from a mixture of cellulose and polyester fibres use was made of aqueous solutions of said salts having a concentration of from 50 to 200 g/l at a pH value of from 2 to 7. After having been impregnated the fabrics were dried. The amount of the flame retardant applied to the fabric depends on its composition and varies from 5% (for cellulose fabrics) to 25% (for fabrics containing up to 50% polyester). A modified fabric did not sustain combustion in the open air. However, a fireproofing effect resultant from the use of said salts is unstable to water treatment procedures.
Selecting one or another method for polymer modification should be determined in each particular case by a required fireproofing efficiency, particularly, it depends on retaining fireproofing characteristics during water treatment procedures (launderings), physical and mechanical properties of the fibres and fibres after treatment, technological particulars and instrumentation of process, as well as technical-and-economical characteristics thereof.